Cell relay networks use high bandwidth, low-delay, packet-like switching and multiplexing techniques for transferring data across the network using fixed sized cells. An example of such a technique is asynchronous transfer mode (ATM), which is a suite of communication protocols designed as a "best efforts" cell delivery system. The fixed sized cells require lower processing overhead and allow higher transmission speeds than traditional packet switching methods. Each ATM cell contains a payload portion and a header, the latter including a label that associates the cell with a logical communication link between sending and receiving network end systems, i.e., the cell's virtual connection or virtual circuit. Thus, the virtual connection label for an ATM cell forms the basis on which the ATM cell is routed, or switched, at each network node it encounters.
The ATM cells are asynchronous in that ATM cells belonging to the same virtual connection do not necessarily appear in fixed time slots at periodic intervals on the transmission medium. Rather, the position of the ATM cells associated with a particular virtual connection is random and depends upon the activity of the network. Thus, an ATM system can automatically adjust the network capacity to efficiently transfer data having significantly different data rates, such as voice, real-time or store-and-forward computer data, Internet data, video services, and so forth.
To maximize the use of shared network resources in cell relay networks, such as ATM networks, these networks desirably are statistically multiplexed. In statistical multiplexing, a communication link is multiplexed between channels on a probabilistic basis. With a large number of bursty connections, all of the connections can be assigned to the same link with a high probability that they will not all burst information at the same time. If they do burst simultaneously, the burst can be placed in a physical buffer until a free time slot becomes available.
At times, cell relay networks can become congested or oversubscribed. When a network becomes over-subscribed, not all cells can be transported by the network, and consequently some cells are discarded. Discarding of cells is an expected phenomena in a cell relay network and generally is accommodated by labeling each cell with a priority.
When a cell relay network becomes congested or overwhelmed with cells that cannot all be transported simultaneously, the network typically discards cells designated with the lowest priority. If the network is still unable to deliver all remaining cells, it then will discard next highest priority cells presented to it. The discarding process proceeds to continually escalating priority cells until the network is able to transmit the remaining cells. Desirably, the cells designated with the highest priority are guaranteed not to be discarded as a result of congestion. This guarantee is achieved by carefully controlling the amount of high priority traffic admitted to the network. For example, when establishing a virtual connection, the party requesting the connection identifies, at the time of connection set-up, the priority of cells that will pass through the connection through use of quality of service (QoS) parameters included in the set-up message. Each such QoS category corresponds to a priority ranging from a highest priority associated with a guaranteed transmission rate to a lowest priority having no associated transmission rate guarantee.
In prior art networks, a subscriber requesting a connection generally specifies a single priority of cells to be transmitted through the network and also the rate at which such cells will be transmitted. While cells can be sent through a virtual connection at varying rates in prior art networks, the priority of such cells is homogeneous. Such a network, therefore, does not enable a subscriber to transmit cells of different priorities through a single virtual connection. Thus, such a network limits the ability to use a virtual connection as a trunk for combining cells of varying priorities.
Such a network also impedes the transmission of traffic using Internet Protocol (IP), particularly with IP version 6 (IPv6), where packets can be designated with up to sixteen different priorities. Therefore, in prior art cell relay networks, a subscriber would need to pay for access to a separate virtual connection for each priority of cell it desires to transmit (necessitating up to sixteen different connections for trunking IPv6 traffic) or subscribe to a single connection which is designated with the highest priority and transmit all traffic over this high priority connection (necessitating overpayment when transmitting low priority traffic over such a connection).
Thus, what is needed are methods and apparatus for facilitating transmission of cells having multiple priorities in a cell relay network through a single virtual connection.